1. Field of the Invention
The present invention relates to an electrical wiring structure, such as a planar coil. More particularly, the present invention relates to an electrical wiring structure, such as a planar coil, capable of being also constructed as a multi-layer structure having a reliable insulation, especially a reliable inter-layer insulation, and a fabrication method of the electrical wiring structure. The present invention also relates to an electrical wiring structure, such as a single-layer planar coil, having a reliable insulation.
2. Description of the Related Background Art
In recent years, electronic equipment has been increasingly reduced in the size and thickness. In synchronization therewith, the density of an electrical wiring on a circuit board and the like tends to increase, and various fine wiring arrangements and fabrication methods thereof have been proposed. Such fine wiring is conventionally fabricated by a method in which a metal thin film is formed by thin-film forming techniques, such as a sputtering method, and a wiring pattern is then formed by etching. In such a fabrication method, a fine structure can be formed if the film thickness is thinned, where the cross-sectional area of the wiring decreases and a resistance of the wiring increases. However, there is a lower limit to the thickness of a thin film in the light of electrical properties of electronic components, and a method of forming an electrical wiring having a large aspect ratio is earnestly desired.
The above-discussed situation also applies to electronic equipment having a driving system. Reduction in the size and thickness of a driving portion is desired, as well as reduction in the size and thickness of a magnetic component, such as an inductor and a transformer.
It should be noted that there is a limit to reducing the size of a conventional wound coil in which a wire is wound on a ferrite core. Accordingly, it is necessary to enlarge the cross-sectional area of a coil and increase the turn number of a coil to achieve a large inductance.
Japanese Patent Application Laid-Open No. 5(1993)-198449 describes a method of fabricating a planar coil on a ceramic substrate with paste using a screen printing. By this method, it is possible to print a thick-film coil on the substrate and achieve a planar coil. However, it is necessary to increase the turn number of the coil and decrease a wiring resistance in order to increase a Q-value of a planar inductor or the like. It is further necessary to decrease a gap between wires and increase the thickness of the wire in order to increase the turn number of the coil and decrease the wiring resistance. In doing so, the aspect ratio of a coil of paste formed by the screen printing is likely to increase. Hence, droop due to fluidity of the paste is likely to occur prior to or at the time of sintering, and a short-circuit is likely to occur between adjacent wires, resulting in a decreased reliability of the coil.
Further, Japanese Patent Application Laid-Open No. 2001-250723 (JPALO-2001-250723) describes another example of a coil having two or more layers as a method of increasing a Q-value of a high-frequency coil. In the coil, the aspect ratio is regulated, and an organic material having a relative permittivity of below 5 is formed as an inter-layer insulating layer with a thickness between 20 microns and 100 microns. JPALO-2001-250723 also describes that the turn number of the coil per unit area can be increased.
However, an increase in the aspect ratio of the coil wiring is required to decrease a resistance of the coil wiring. In addition, the thickness of an inter-layer insulating layer should be decreased to enhance the magnetic flux density of a coil. In the method of JPALO-2001-250723, therefore, step coverage of the insulating layer is likely to become bad at the outermost periphery of the coil, and there is a high possibility of occurrence of a short-circuit when another coil or the like is layered on this insulating layer. Thus, there is decreased reliability of the inter-layer insulation.